- Age 60 or older; or age 18 or older with gastrointestinal abnormalities including but not limited to gastrointestinal surgery (e.g. gastrectomy, gastric bypass), ileal resection, gastric atrophy, Celiac disease, Crohn's disease, or prolonged use (>3 months) of proton pump inhibitor drugs, or on a restricted diet (such as vegetarian or vegan).